VHE Gamma-ray Astronomy

The Very Energetic Radiation Imaging Telescope Array System (VERITAS), the new telescope array proposed here, represents a dramatic step forward in the study of extreme astrophysical processes in the universe. It will substantially increase the catalog of very high energy (VHE, E $\mathrel{\hbox{\rlap{\hbox{\lower4pt\hbox{$\sim$ }}}\hbox{$>$ }}}$100GeV) gamma-ray sources and greatly improve measurements of established sources. The field of ground-based gamma-ray astronomy has been revolutionized by the power of the atmospheric Cherenkov imaging technique for the discovery and study of individual sources. This technique was largely developed by the Whipple Observatory Gamma-Ray Collaboration. Although less than 1% of the sky has been surveyed by the imaging technique at 300GeV, thirteen sources have now been reported (eight with high significance) by ground-based groups using the imaging technique: three pulsar-powered nebulae, six BL Lacertae-type active galactic nuclei (AGN), three shell-type supernova remnants, and one X-ray binary system ([Weekes 1999]). These measurements have advanced our understanding of the origin of cosmic rays, the nature of AGN jets, the density of the background infrared (IR) radiation, and the magnetic fields in the shells and nebulae of supernova remnants.

Gamma rays are unique probes of the Universe. Unlike radiation at longer wavelengths, gamma-rays are not attenuated significantly within the Galaxy. As such, they provide an unobstructed view through the plane of the Galaxy except for regions which contain high densities of low energy photons (or virtual photons in regions of high magnetic fields) where VHE gamma-rays are attenuated via photon-photon pair production. Thus, the regions where the high energy gamma-rays are emitted must have relatively low photon densities (e.g., in active galactic nuclei) or low magnetic fields (e.g., near a pulsar light-cylinder). For gamma-rays from extragalactic sources, pair production with low energy photons can be used to probe intergalactic radiation fields. Finally, VHE gamma-rays are typically at or near the end of the electromagnetic emission spectra of the objects which produce them, so the VHE gamma-ray emission characteristics are often the most sensitive to the parameters used in the emission models and they probe the most energetic range of the underlying physical processes.